The present invention relates generally to improved methods for designing fracturing programs for fracturing subsurface formations, and more specifically relates to improved methods for utilizing small scale test fracture operations and analysis, commonly known as "mini-frac" operations, to design subsurface formation fracturing programs.
Mini-frac operations consist of performing small scale fracturing operations utilizing a small quantity of fluid, which typically contains little or no proppant. After the test fracturing operation, the well is shut-in and the pressure decline of the formation is observed over time. The data thus obtained is used in a fracture model to establish parameters to be used in designing the formation fracturing program.
Mini-frac test operations are significantly different from conventional full scale fracturing operations in that only a small amount of fracturing fluid is injected, for example, as little as about 25 barrels, and no significant amount of proppant is typically utilized. The desired result is not a propped formation fracture of practical value, but a small scale, short duration fracture to facilitate collection of pressure decline data in the formation. This pressure decline data will facilitate estimation of formation, fluid and fracture parameters.
A major limitation on the value of conventional methods of mini-frac analysis is that the methods rely on assumptions that the fracturing fluid is both incompressible and isothermal. Conventional techniques thus ignore the significant effects which may be presented by compressibility of the fracturing fluid and by temperature increase of the fracturing fluid. For example, use of conventional mini-frac techniques with the Perkins and Kern model has been found to lead to an error in calculated fluid loss coefficient of up to 100%, and to an error in calculated fracture length of up to 75%. The assumption of an incompressible fracturing fluid may lead to particularly erroneous results where foam is used as a fracturing fluid. When the effects of fluid compressibility and temperature increase are considered, the determined fracture length typically decreases while the determined leakoff coefficient and average fracture width typically increases.
Accordingly, the present invention overcomes the deficiencies of the prior art and provides a new method for mini-frac analysis wherein the compressibility of the fracturing fluid and the increases in fracturing fluid temperature are considered, thereby facilitating the designing of optimal subsurface fractures.